A reciprocating saw machine is a hand-held power saw that includes a chuck for releasably engaging the saw blade and driving the saw blade in a reciprocating motion through a work piece. The reciprocating motion can be an orbital cutting action, a straight or linear cutting action, or an angled cutting action. The length or stroke of the reciprocating motion is typically about 1½ inches or less. Reciprocating saws are sometimes referred to as “recip” saws, jig saws, and power hack saws, and the term reciprocating saw is used herein without limitation to mean reciprocating saw machines, jigsaw machines, and portable power hack machines. Reciprocating saws typically are driven by electric motors (e.g., cord or cordless saws) or are pneumatically driven. Well-known reciprocating saws are sold under the brand names “Sawzall™” by Milwaukee Electric Tool Corporation and “Tiger Saw™” by Porter-Cable Corporation.
A typical reciprocating saw blade includes a blade portion having a cutting edge defined by a plurality of teeth axially spaced relative to each other along one side of the blade, and a non-working edge formed on an opposite side of the blade relative to the cutting edge. A tang for releasably connecting the blade to the chuck of a reciprocating saw extends from an inner end of the blade. The term “recip blade” or “reciprocating saw blade” is used herein to mean a blade configured for use in a reciprocating saw, but is not limited to any particular configuration of blade or use in a particular saw.
A typical reciprocating saw blade intended for cutting soft materials such as wood, including composite or bi-metal blades, is designed to cut fast and aggressively. Aggressive cutting tooth forms along with a large pitch (typically 2 to 8 teeth per inch) are used for this purpose. However, such blades are susceptible to failure upon encountering an occasional hard material, such as a, nail or screw (typically having a diameter of at least about 40% of the tooth pitch) or staple when the hard material falls too far into a tooth gullet beyond the end of a tooth tip. This type of failure can also occur with the cutting of pipes or materials where the cut cross-section changes depending on the blade's location within the cut e.g. on a pipe where the cut cross-section is wide at the top and then is drastically reduced as the saw approaches the cross-section that is perpendicular to the cutting direction. This could be the side walls of a round pipe, a rectangular tube or any structural work piece. If the wall thickness (or the dimension of the material in the cutting direction) becomes less than the tooth pitch, the saw could overfeed. This “over-feeding” of the hard material, forces the trailing tooth to cut a large portion of the hard material, thus forcing a bigger chip load than the trailing tooth can handle. Under these circumstances, the tooth may not withstand the resultant shearing force, resulting in fracture. Additionally, saw stalling may be induced, leading to injury.
Similarly, specialty reciprocating saw blades, such as diamond or carbide tipped blades, are very effective when used for their intended purposes, but perform very poorly if misapplied. The material at the tip of these blades possesses a higher hardness than a typical bi-metal blade, and consequently is also more brittle. This renders such blades susceptible to catastrophic failure when they come in contact with a hard material, such as a pipe, nail, screw or staple, due to their brittleness. In such instance, the tip may fracture, crumble, or rip from the weld or solder with the blade body.
Prior art attempts to solve the problem of tooth fracture upon encountering hard materials include employing blades with varying shallow clearance angles (between 17 degrees and 23 degrees) on alternate teeth, or employing tooth shapes having humps at the end of the primary clearance surface, to prevent hard materials, such as pipes, nails, screws or staples, from falling within the gullet and causing tooth fracture. However, such shallow clearance angles sacrifice cutting efficiency and the life of the blade in exchange for some potential prevention of tooth fracture. Further, the extension of the clearance surface to form the humps can reduce gullet volume, reducing chip removal capacity/efficiency, or require a larger tooth pitch, which reduces cutting capacity/efficiency or requires modification of the teeth to make up the loss, which can further exacerbate the problem of breakage when encountering hard materials.
Prior art attempts to solve this problem for diamond or carbide tipped blades include designing different types of pockets along the blade body for the diamond or carbide tips to reside in, adjusting the parameters for welding and soldering the tips to the blade backing, as well as employing different material grades to impart different shock and impact absorption properties. However, none of these configurations prevent the underlying problem of over-feeding.